The advantages of using an all-wheel drive system in a vehicle are well known. In general, an all-wheel drive system improves traction, and thus safety, by allowing power to be sent to all four wheels, rather than just the front two or the rear two. Thus when traction is lost in one or more wheels, for example due to wet or icy road conditions, the drive system can increase the torque to the axle/wheels with traction.
A variety of control systems have been developed to detect tire slippage, i.e., wheel spin, and to redirect the available torque to the remaining wheels. These control systems range from simple hydraulic and mechanical systems to relatively sophisticated electronic control systems. For example, U.S. Pat. No. 4,589,511 describes a traction control system that uses wheel spin sensors to detect the spinning of a wheel or wheels, and an electronically controlled wheel braking system to prevent wheel spinning.
Many of the current traction control systems, while providing efficient traction control in a conventional vehicle utilizing a combustion engine drive train, are unsatisfactory for a hybrid or all-electric vehicle due to differences in vehicle weight and weight distribution, and more importantly differences in drive train torque and power capabilities. Accordingly, what is needed is a traction control system designed to meet the needs of such alternative fuel vehicles in general, and all-electric vehicles in particular. The present invention provides such a system.